1. Field of the Invention
The present invention relates to the technology of prebaking of alignment film, and in particular to a device for prebaking an alignment film by using a temperature-controllable pin to support a substrate and a method thereof.
2. The Related Arts
Prebake of alignment film (also referred to as PI) is a process carried out after application of PI material to a substrate (by means of coating or inkjet printing) and before main cure to volatize solvent contained in the PI solution (which process volatilize 80% solvent). The primary constituent components of the PI solution are polyimide, N-methyl-s-pyrrolidone (NMP), and ethylene glycol butyl ether (BC). The purpose of prebake is to volatilize and remove the solvents, such as NMP and BC, from the PI solution in order to make the PI film flat and smooth. The primary measure adopted in this industry for prebaking of PI is heating a substrate with infrared radiation so as to cause volatilization of the solvent. Major process problems of such a baking technique include non-uniform heating and non-uniform volatilization in occurring in the baking process. The first problem is that contact of support pins with the substrate causes inconsistent heating between sites where contact is made and sites where no contact is made and this leads to “pin mura” caused by the pins. The second problem is that the volatilization speeds of solvent at a peripheral area and a central area are different and this leads to area of non-uniform film thickness, which is referred to as a “halo area” or “edge area”. Consequently, product quality is affected.
The conventional process adopted in the industry is prebake plus proximity pin. As shown in FIG. 1, which schematically shows pin mura generated in the conventional process of prebake plus proximity, circular areas of a prebaked substrate 1 contain pin mura 10. The cause of the generation of the pin mura is that the proximity pins that support the substrate are generally not absolutely insulating. This causes inconsistent heating between the supported portions and the non-supported portion in the prebaking process and eventually affects the homogeneity of the volatilization of the PI solution on the surface of the substrate, leading to the pin mura formed on the surface of the substrate.
Conventionally, there are generally two measures that are adopted to alleviate pin mura. One is to improve the material of the pins by using material having better thermal insulation property, such as polyether ether ketone (KEEP), or a new substance polysalfone (PSF). The other is to improve the way of supporting of the proximity pins, which includes “lift pin” and “move pin”. As shown in FIG. 2, which demonstrates the operation of the conventional way of supporting with “move pin”, a substrate 2 is supported by movable pins 21 and fixed pins 22. The movable pins 21 are movable to change the positional status of the substrate 2, such as original point, ascending, advancing, descending, and reversing. As shown in FIG. 3, which demonstrates the operation of the conventional way of supporting with “lift pin”, a substrate 3 is raised alternately by lift pins 31 and lift pins 32. The structures that can effect the operations of supporting illustrated in FIGS. 2 and 3 are known to those having ordinary skills in the art and thus detailed description is omitted herein. These operations, although to certain extents alleviating pin mura, yet are generally not possible to totally eliminate pin mura.